Switch mode regulators are well known and are used to convert one DC voltage output to another. An example of a switch mode regulator is shown generally at 10 in the schematic illustration of FIG. 1, and comprises a first switch 12 which controls a current supply to an output inductor 14. A second switch 16 is provided for selectively connecting the output inductor 14 to ground, whilst an output capacitor 18 smoothes the output voltage provided by the output inductor 14.
In operation of the switch mode regulator 10 the first switch 12 is switched on and off by a control circuit 22 to alternately connect and disconnect the inductor 14 to a supply voltage, with the resulting current flowing through the inductor 14 into a load 20. The ratio of the time for which the first switch 12 is switched on to the time for which the first switch 12 is switched off is known as the duty cycle.
It is commonplace to use a switch mode regulator 10 in a discontinuous current mode of operation when a low output current is required. In this mode of operation, the current through the output inductor 14 falls to zero for part of the duty cycle of the regulator 10.
When operating in discontinuous mode the output of the regulator has three distinct states:
1) The first switch 12 is switched on. In this mode positive current flows from the supply through the inductor 14 into the load. The current ramps from zero to a peak current;
2) The first switch 12 is switched off and the second switch 16 is switched on. In this mode the voltage at the output of the first switch 12 is zero, and a positive current flows from ground through the switch 16 and the inductor 14 into the load 20. This positive output current ramps down to zero;
3) When the current flowing through the inductor 14 drops to zero both the switches 12, 16 are turned off and the voltage at the input of the inductor 14 is equal to the voltage at the output of the inductor 14. The regulator 10 is in a high-impedance state.
The duration of the first state is defined by control circuitry controlling the switch mode regulator 10.
The start of the second state is defined by the point at which the second switch 16 is turned on. It is important that the timing of this transition is accurate relative to the time when the first switch 12 is turned off. If it is too early, both switches are on at the same time and a current path is provided between supply and ground leading to undesirable losses. If it is too late, a high back-EMF (electromotive force) will be generated across the inductor which can lead to undershoot, excessive voltages, damage to the second switch or other circuitry, and/or undesirable losses.
The end of the second state is defined by the point at which the current flowing through the inductor drops to zero, which is dependent upon external factors and is difficult to predict. Thus, in order to determine the duration of the second state, some means (indicated at 24 in FIG. 1) must be provided for detecting when the current flowing through the inductor 14 drops to zero.
It is important that this detection is accurate, since the detected point at which the current flowing through the inductor 14 drops to zero triggers the third state. If the detection is inaccurate and the second switch 16 is turned off too early or too late a high back-EMF (electromotive force) will be generated across the output inductor, which can lead to overshooting of the desired inductor input voltage and subsequently to ringing at the input of the output inductor 14, damage to either or both of the switches 12, 16 and/or undesirable power loss.
Thus, it is important that the detection of the zero-crossing point (i.e. the point at which the current flowing through the output inductor drops to zero) is fast and accurate. Additionally, to avoid degrading the efficiency of the regulator it is desirable for any additional detection circuitry to have a low quiescent current.